1. Technical Field
The present invention relates to a ferroelectric memory device, and an electronic apparatus and a driving method using the ferroelectric memory device.
2. Related Art
Japanese Laid-open Patent Application JP-A-2004-241097 describes a ferroelectric memory and its initialization method. The aforementioned document describes a method for improving depolarization of a ferroelectric capacitor which occurs due to thermal history during packaging, in which one cycle of a reference plate line is driven only at the time of first power-on.
A ferroelectric memory (FeRAM: Ferroelectric Random Access Memory) device stores information, using its hysteresis characteristic observed between polarization of the ferroelectric material and electric field. Ferroelectric memories are attracting attention in view of their high speed operation, low power consumption and nonvolatility, and are being studied for their application to various devices. For example, a microcomputer (micro-computer) may be provided with a program region (ROM (read only memory) region) and a RAM (random access memory) region; and as a ferroelectric memory is non-volatile and its data is re-writable, both of the regions can be formed from ferroelectric memories.
On the other hand, when a packaged ferroelectric memory (semiconductor chip) is assembled in an electronic apparatus, for example, when it is mounted on a wiring substrate composing the electronic apparatus with solder or the like, a thermal load (heat stress) of about 260° C. would be applied to the ferroelectric memory.
When such a thermal load is applied, the polarization state of the ferroelectric film would be depolarized. Therefore, according to the document described above, the depolarization is improved by driving a reference plate line by one cycle at the time of first power-on.
However, when the depolarization is excessive, in other words, when the depolarization occurs to the extent that previously written data becomes inverted, its relief is not possible even by using the technique described in the aforementioned document.